SaaS user here too...
To do this you have to cancel the connected incoming delivery first.
In the confirmed PO, go to 'Receive Products' and cancel the incoming delivery. Navigate back to the PO and click 'Cancel'. You now see the 'Set to Draft' that Fabrice mentions.
When you only have to change dates or availability on 1 or a few items, there's another way:
Go to 'Warehouse', left menu 'Receive/Deliver Products' and 'Incoming Products'. Here you can filter on PO or product. You can cancel an individual move (it will show as canceled in the incoming delivery) or change the 'expected' date.
I like this feature as it allows me to change dates or availability of a product that were ordered across several PO's.